1. Field of the Invention
This invention relates to a head control apparatus enabling the head to seek tracks stably at high speed in record/reproduce apparatuses with a replaceable rotating recording medium, such as CD-ROM readers, optical disk apparatuses, large-capacity floppy-disk apparatuses, or removable hard disk apparatuses.
2. Description of the Related Art
Record/reproduce apparatuses (or reproduce apparatuses) using a replaceable rotating recording medium include magnetooptic disk apparatuses (or CD-ROM apparatuses). Magnetooptic disks (or CD-ROM disks) used in those disk apparatuses have concentric or spiral information tracks. In the disk apparatuses, magnetic characteristic-changed regions, metal state-changed regions, or physical irregularities (pits) are formed on the information tracks or between adjacent information tracks, and the information is written (recorded) into or read (reproduced) from those regions.
For example, in magnetooptic disk apparatuses that record and reproduce the information along the tracks, the optical head is controlled so that the light spot may be centered directly on the track. When the information is written into or read from the target track away from the current optical head position, the head tries to access the target track. The action of accessing the track is called the seeking operation.
In the seeking operation, by moving the head carriage assembly supporting the optical head in the direction of the radius of the optical disk under speed control, the light spot from the optical head moves swiftly to the target track, traversing some of the concentric tracks.
Such speed control requires sensing of the head moving speed. The head moving speed can be sensed by digitizing the readout signal from the optical head (for example, the push-pull combined signal from the output of two photo diodes) on the basis of a specific level, then counting the interval between the digitized signals in response to a specified clock, and finally computing the reciprocal of the count result, which represents the head moving speed.
To enable the light spot to cross concentric tracks and move to the target track quickly and reliably, it is necessary to sense the head moving direction (the head's motion from the internal to the external circumference or vice versa) in the speed control. The reason for this will be explained below.
It is assumed that the light spot moves from the external to the internal circumference of the rotating disk. When a disk is off-centered, the concentric tracks on the disk move back and forth in the direction of the disk's radius at a frequency proportional to the rotating speed of the disk. When the speed at which the tracks move toward the internal circumference of the disk exceeds the moving speed of the light spot, the relative speed between the light spot and the tracks with respect to the disk's radial direction is temporarily inverted in sign.
Specifically, despite the fact that the light spot is moving toward the internal circumference of the disk, there arises the situation in some portions where the light spot moves toward the external circumference of the disk. That is, the direction of movement the light spot varies with respect to the disk, due to the disk's eccentricity.
Therefore, in an optical disk apparatus using a replaceable rotating recording medium that is inevitably off-centered to some extent, the ability to sense the direction in which the light spot is moving is necessary for performing quick and reliable speed control.
The direction of the light spot movement can be detected from the phase difference between the digitized difference signal (the push-pull combined signal) of the opposite phase output from a plurality of optical sensors constituting the optical head and the digitized sum signal of the in-phase outputs.
Specifically, as shown in FIG. 5, by sampling and holding the signal level L1 of digitized difference signal SD17 at the rising edge of digitized sum signal SA19, it can be sensed with L1=high level that the light spot moves from the external to the internal circumference.
As shown in FIG. 6, by sampling and holding the signal level L2 of digitized difference signal SD17 at the rising edge of digitized sum signal SA19, it can be sensed with L2=low level that the light spot moves from the internal to the external circumference.
when the phase difference becomes almost 0.degree. (the in-phase) or 180.degree. (the opposite phase) for some reason, however, the moving direction cannot be sensed.
Specifically, as shown in FIG. 7, when the rising edge of digitized sum signal AS19 has overlapped with the signal edge of digitized difference signal SD17, even after the signal edge level LX of digitized difference signal SD17 has been sample-held, it is not determined whether LX is at high level or low level, so that it is impossible to sense the direction in which the light spot is moving.
The sum signal has a relatively small amplitude as compared with the difference signal whose amplitude is easily made larger by performing the push-pull operation. When the readout signal level of the sum signal is so low that the digitized output of the sum signal cannot be obtained, it is impossible to sense the moving direction.
Specifically, as shown in FIG. 8, when no digitized sum signal SA19 is produced and the digitized difference signal SD17 cannot be sampled and held, it is impossible to sense the direction in which the light spot is moving.
when the speed control is carried out in the situation where it is impossible to sense the direction in which the light spot is moving, not only will high-speed, accurate access to the target be impossible, but also a running away of the control system will make the seeking operation unstable.
Since the movement of the tracks due to the eccentric rotation of the disk takes the form of a sinusoidal wave, when the average is taken over a period of time longer than the period of the sinusoidal wave, relative changes in the seeking direction due to the movement of the tracks are canceled, thus making it possible to perform speed control provided that the seeking direction is not inverted. To achieve this, the seeking speed must be sufficiently slow as compared with the speed at which the tracks move back and forth in the radial direction of the disk.
If the operating parameters of the control system are set so that the seeking operation is very slow, due to the fear that the control system may become unstable, the high speed track seeking ability of the head will be sacrificed.